Numerous locations in the world have been affected by chemical releases to the subsurface. These chemical releases often contaminate ground water in the vicinity of the chemical release, which may negatively impact human populations and wildlife. The contamination problem is especially problematic in industrialized nations where storage and transport of chemicals is widespread. However, in many nations work is ongoing to clean up the contaminated ground water using a number of remediation technologies.
One promising technology for clean up of ground-water systems is in-situ detoxification of the contamination by microorganisms. This technology has been proven effective for a number of contaminants including petroleum hydrocarbons, fuel additives such as ethanol and methyl-tertiary butyl alcohol, and chlorinated organic compounds. The contaminant compounds are processed metabolically by the microbes and transformed into innocuous end products. Two classes of compounds are required for bacterial metabolism; electron donors and electron acceptors. Microbes generate energy through an internal enzymatic process that couples consumption of electron donors with respiration of electron acceptors. Many contaminants amenable to bioremediation can be characterized into one of these two categories. The most common electron donor contaminants are petroleum hydrocarbons, which include the solvents benzene, toluene, ethylbenzene, and xylene. Common electron acceptor contaminants include nitrate, perchlorate, and chlorinated hydrocarbons like perchloroethene, trichloroethene, and carbon tetrachloride.
Bioremediation can occur naturally, however, it is often preferential to increase the rate of bioremediation by modifying the subsurface environment to be more favorable for biological activity. In order to do this, harmless chemical compounds that stimulate biological uptake of contaminants are added to the contaminated aquifer. This is typically accomplished by injecting water-based solutions of the required compounds into the contaminated aquifer.
Electron Acceptors
Electron acceptor compounds are added to aquifers impacted by electron donor compounds. The most energetic biological reactions are aerobic reactions that occur when microorganisms couple respiration of oxygen, which is an electron acceptor, with consumption of an electron donor. Because aerobic microbial reactions are more energetic than respiration of anaerobic electron acceptors such as nitrate, sulfate, and carbon dioxide, aerobic bioremediation progresses at a faster rate than anaerobic bioremediation. Additionally, injecting compounds such as nitrate and sulfate into the aquifer can result in a secondary form of contamination (i.e. nitrate and sulfate contamination) if electron acceptor uptake is incomplete.
Electron Donors
It is desirable to add electron donor compounds to aquifers contaminated with electron acceptor compounds. There are a number of electron donors that can be used for bioremediation. Suthersan (2003) describes a system where solid and liquid carbohydrates such as sugar, molasses, fruit juices, and sucrose are dissolved in water and added to the subsurface as an electron donor. Newell (1997) similarly describes a method for bioremediation by which hydrogen in gaseous or dissolved form is added to the subsurface. The hydrogen stimulates microbial activity and increases the rate of electron acceptor uptake.
Electron donors and electron acceptors may be added simultaneously to induce cometabolic degradation of contaminants. One of the common injection combinations is injection of dissolved oxygen as an electron acceptor and dissolved methane as an electron donor. These compounds stimulate a class of microbes called methanotrophs. In addition to consuming oxygen and methane introduced into their environment, contaminant compounds are broken down by broad activity enzymes produced by the methanotrophs.
Fouling in Bioremediation Systems
Solutions of electron acceptors and electron donors are typically added to the subsurface through a network of injection wells, infiltration trenches, or leach fields. A primary maintenance concern for remediation systems that are based on water injection into aquifers is plugging of the injection network. A significant cause of fouling in enhanced bioremediation injection systems is growth of bacteria within the injection borehole and/or infiltration trench and the surrounding aquifer. This type of fouling is referred to as biofouling. Implementing a mechanism to reduce the growth and proliferation of microorganisms responsible for the biofouling can reduce maintenance costs due to biofouling.
Sterilization of the injection well and surrounding aquifer materials is one strategy for reducing biofouling.
It would be desirable to employ ozone as a biocide because ozone is a very effective biocide that, when dissolved in water at milligram per liter concentrations, destroys most microorganisms by lysing their cellular membranes through oxidation reactions.
However, delivery of ozone to the subsurface is problematic due to the reactive nature of ozone. Autocatalytic destruction of ozone occurs once it is dissolved in water, which means its effectiveness as a biocide decreases over time. The ozone half-life is dependant on a number of factors including the concentration of dissolved ozone and dissolved organic and inorganic species in the contact water. The ozone half-life, which is typically on the order of minutes, decreases as the concentration of dissolved constituents increases. As a result, the half-life of ozone is greater in distilled water than typical municipal water, and in turn greater in typical municipal water than a typical ground water. In heavily contaminated ground waters the half-life of dissolved ozone is too short to be measured by standard techniques.
The inventor determined the half-life of ozone in typical municipal water by using standard field techniques. Ozone was dissolved in municipal water and the concentration of dissolved ozone measured over time. FIG. 1 is a graph of the ozone data. From this experiment it was determined that the half-life of the ozone in the representative municipal water was approximately 5 minutes. A second experiment was conducted where the ozone gas was dissolved in petroleum-impacted ground water. Though the ozone contacting process was identical, no ozone was detected in the impacted ground water as a result of instantaneous depletion of the reactivity of the ozone, which was quenched through spontaneous reactions with the dissolved petroleum compounds. Because most ground-water injection remediation systems centralize process equipment in one location and distribute injectant from that central location to the injection well/infiltration trench network, the residence time of the injectant in the piping network can be on the order of hours. In this type of system, the reactivity of the beneficial ozone is depleted by the time ozone amended water reaches the injection network. In order for the ozone to be effective as a biocide, the gas/liquid contacting process must occur at the point of application rather than at the centralized equipment location.
Gas Contacting with High Purity Gas
Adding gasses to a contaminated aquifer to enhance bioremediation is problematic due to the low solubility of many gasses in water at common temperatures. The maximum concentration of dissolved oxygen in water in contact with atmospheric air at 15 degrees Celsius and low salinity is 10.1 milligrams per liter (mg/L). The amount of dissolved oxygen that must be introduced into the subsurface to completely remediate the target contaminants is a function of the mass of contamination to be treated and the amount of oxygen required for full oxidation of the target contaminants. For petroleum hydrocarbons, the stoichiometric oxygen requirement is three to four moles of oxygen for biological oxidation of one mole of petroleum hydrocarbon. In addition mass of the oxygen required to remediate the hydrocarbons, it is typical that an excess of oxygen must be supplied to satisfy abiotic oxygen demands. Therefore, for an extensive petroleum hydrocarbon release, the volume of air saturated water that must be introduced to the subsurface to satisfy the biological oxygen requirement for transformation of the hydrocarbons can number in the millions or billions of liters.
The total volume of injectant required for bioremediation can be reduced by increasing the dissolved oxygen concentration of the injectant. Dissolution of gasses in liquid is achieved by contacting the gasses to be dissolved with the liquid. The final concentration of a dissolved gas is dependant on the volume of gas in the contacting process, the efficiency of the contacting process, and the ratio (partial pressure) of the gas in the contacting mixture. Assuming that an excess volume of gas is provided to a 100 percent efficient contacting process, the final concentration of gas in liquid is dictated by Henry's law, which can be expressed as:S=kHP                S=solubility of the gas (mol/L)        kH=Henry's law constant [(mol/L)/atm]        P=partial pressure of the gas (atm)        
Air is a mixture of a number of gasses. The primary components of air are nitrogen and oxygen, which account for approximately 78 and 21 percent, respectively, of the total gas. As discussed above, the maximum concentration of dissolved oxygen in contact with air at 15 degrees Celsius is 10.1 milligrams per liter. Increasing the ratio of oxygen in the contact gas to 100 percent increases the partial pressure of the oxygen in contact with the water by approximately five-fold. Therefore, approximately 48 milligrams per liter of oxygen can be dissolved in water at 15 degrees Celsius.
Increasing the concentration of dissolved gas in a contacting process by using high-purity gas is relatively straightforward. However, gasses used for remediation such as oxygen, methane, and propane have costs associated with them. Because large volumes of these gasses are required for the aquifer remediation strategies discussed above, it is advantageous to conduct gas/liquid contacting processes as efficiently as possible to reduce the amount of gas wasted in the process and, as a result, overall cost.
Gas Contacting Processes
There are a number of methods for gas/liquid contacting for low viscosity liquids. Among the traditional gas/liquid contacting methods are bubble columns, stirred vessels, static in-line mixers, U-tube columns, and plunging jets. A refinement of the plunging jet gas contacting method is a confined plunging liquid jet (CPLJ) contactor. Confined plunging liquid jets are described in the articles, Evans, et al., Mass Transfer in a Confined Plunging Liquid Jet Bubble Column, Chem. Eng. Sci. 54 (1999) 4981-4990, and Jakubowski, et al., Ozone Mass Transfer in a Confined Plunging Liquid Jet Contactor, Ozone Science and Engineering, Vol. 25, pp. 1-12 (2003), both of which are incorporated herein by reference in their entirety.
In this type of contactor, a high-speed stream of liquid is generated by passing pressurized liquid through an orifice smaller than the upstream pipe diameter.
FIG. 2 shows an embodiment of the present invention employing a CPLJ. In this embodiment pressurized liquid passes through an orifice 3 which is vertically oriented and creates a high velocity jet of fluid 4 that impinges into a body of fluid located beneath the orifice 3. Gas is either injected into the liquid upstream of the orifice or is drawn into the process at the point of impingement (FIG. 2, showing an embodiment of the present invention). The plunging jet impinges into a body of fluid that is confined by a downcomer 6. Near the point of impingement is a highly energetic, turbulent zone where the downward force of the plunging jet fights buoyancy forces of the entrained gas. This zone, called the mixing zone 8, is characterized by vigorous mixing of the gas and liquid and high gas to liquid surface area due to the small gas bubble size created by the impinging jet. It is in this zone where the bulk of the high-efficiency gas/liquid contacting occurs. Below the mixing zone is a zone called the pipe flow zone 9. This zone 9 is characterized by a less turbulent flow pattern where the liquid and excess gas flow downward and exit the downcomer 6.
Several researchers have studied CPLJ gas/liquid contactors. In one such study, the researchers measured mass transfer coefficients for a laboratory-scale CPLJ contactor. The researchers used these measurements to develop a mathematical description of mass transfer of carbon dioxide into water within a CPLJ (Evans, 1999). The empirical formula that they developed is expressed as:
            (                        k          L                ⁢        a            )        MIX    =      0.467    ⁢          ɛ      diss      0.207        ⁢          u              G        ,        in            0.697                      (kLα)MIX=mass transfer coefficient [1/s]        εdiss=energy dissipation per unit volume [W/m3]        uG,in=gas feed rate [m/s]        The energy dissipation is described by the expression:        
      ɛ    diss    =                              ρ          L                ⁢                  u          N          3                            2        ⁢                  L          MIX                      ⁡          [              b        -                  2          ⁢                      b            2                          -                                            b              3                        ⁡                          (                              1                -                λ                            )                                2                +                  2          ⁢                                    b              3                        ⁡                          (                              1                +                λ                            )                                          ]                      ρL=density of the liquid [kg/m3]        uN=liquid velocity at the nozzle [m/s]        LMIX=length of the mixing zone [m]        
  b  =            F      N              F      C                      FN=cross-sectional area of the nozzle [m2]        FC=cross-sectional area of the column [m2]        
  λ  =            Q              G        ,        in                    Q      L                      QG,in=gas feed rate [m3/s]        QL=liquid feed rate [m3/s]        
Though this empirical description is derived based on a small sampling of laboratory-scale CPLJ contactors, it gives valuable insight into the factors that control CPLJ efficiency. From this equation, it is apparent that the velocity of the impinging jet, which is a function of liquid flow rate and nozzle size, the diameter of the downcomer, and the length of the mixing zone are the major factors that control contacting efficiency.
The present technology typically uses a large gas/liquid contacting chamber to dissolve gasses into water for injection. The large gas/liquid contactors of the present art are sized to process all injection water at a remediation site and create a uniform dissolved gas concentration in that injection water, which is then passed though a manifold that routes the water to the injection wells.